This invention relates to spray drying apparatus wherein a substance to be dried is introduced into a heated, flowing, drying medium in the presence of acoustic energy to significantly reduce the moisture content of the substance being processed. More particularly, this invention relates to improvements in spray drying apparatus of the type wherein the substance to be dried is injected into the exhaust stream of a pulse jet engine.
U.S. Pat. No. 3,618,655, issued to Raymond M. Lockwood and assigned to the assignee of the present invention, discloses a spray drying system in which five pulse jet engines are mounted in fluid communication with a cylindrical drying tank to supply the heated, moving gaseous flow and broad band acoustic energy that atomizes and dries a substance being processed. In this arrangement, each pulse jet engine is of a conventional U-shaped geometry wherein the engine exhaust pipe is curved so that the engine exhaust gases are turned through an angle of 180.degree., i.e., the engine air inlet orifice faces in the same direction as the exhaust orifice. One of the engines, which has come to be known as the "feed" engine, is mounted below the floor of the cylindrical drying tank with the exhaust pipe thereof extending through the tank bottom and the inlet orifice of the engine inlet pipe being mounted in spaced apart juxtaposition with a cylindrical air augmenter that extends upwardly through the tank floor. The substance to be dried is introduced into the exhaust stream of the feed engine by a feedpipe that passes coaxially along the interior of the feed engine exhaust pipe and terminates a short distance from the end of the engine exhaust pipe.
The four additional U-shaped engines of the system disclosed by Lockwood are mounted at equally spaced apart positions around the lower portion of the tank, with the inlet and exhaust pipes thereof passing through the tank wall. More specifically, these four engines are mounted in oppositely disposed pairs with one pair of engines being substantially horizontal to and tangentially intersecting the wall of the tank to induce hot gaseous flow in a circumferential direction. The engines of the second pair are inclined at approximately 30.degree. relative to the horizontal and are mounted to direct gaseous flow inwardly into the tank along paths that form chords which intersect the drying tank axis of symmetry approximately midway between the tank center and sidewall.
Each of the four additional pulse jet engines utilizes cylindrical or ring-like air augmenters that are spaced apart from the engine inlet and exhaust openings so that cooler ambient air is entrained with the hot engine effluent and carried into the tank. Through the use of this secondary airflow, the temperature at the tank floor is maintained at approximately 400.degree. F. Further, the gaseous flow of the four additional pulse jet engines establishes turbulent air currents within the tank which are described in the Lockwood patent as being a series of concentric cyclone-like vortices having alternate upward and downward directions of flow. In order to stabilize these flow vortices about the vertical axis of the tank, the apparatus disclosed by Lockwood includes a centrally mounted horizontal plate at the top of the tank and a truncated cone that extends upwardly from the center portion of the tank floor at a position adjacent to the air augmenter of the feed engine.
In the apparatus disclosed by Lockwood, the interior of the drying tank is maintained at a slightly positive pressure and a slurry or pumpable solution of the material to be dried is injected into the exhaust stream of the feed engine. The slurry is atomized by the hot exhaust stream of the feed engine with the high temperature of the exhaust gas and broad band acoustic energy contained therein removing approximately 15 to 20 percent of the moisture as the injected material passes between the terminus of the feedpipe and the bottom wall of the drying tank. As the material is carried into the drying tank, it is circulated around the interior of the tank by the air currents supplied by the four additional pulse jet engines. Since, in the Lockwood apparatus, the temperature within the tank varies between approximately 400.degree. F. at the bottom of the tank to 200.degree. F. at the top of the tank, the material continues to dry as it is circulated throughout the tank. As the material dries, the lighter particles pass outwardly through an opening in the top of the tank and into a conventional cyclone separator. The heavier particles fall to the bottom of the tank and are removed through openings. In this respect, the Lockwood patent discloses that when fishmeal is being processed, approximately one-third of the fishmeal is carried out through the opening in the upper portion of the tank while the remaining two-thirds settles to the bottom of the tank.
Although the drying system disclosed in the Lockwood patent provides satisfactory operation in many respects, several disadvantages and drawbacks are encountered. First, the use of one or more feed engines in conjunction with the four additional engines that are mounted about the lower periphery of the tank requires a substantial amount of fuel relative to the amount of drying effected. In this respect, the Lockwood patent notes that one such system utilized a 2,000,000 BTU per hour feed engine, with the four additional engines each being rated at 1,000,000 BTU per hour. In this particular system 4,000 pounds of raw fish were processed per hour to yield about 1,000 pounds of fishmeal. Thus, 6,000,000 BTU per hour are required to evaporate approximately 3,000 pounds of water and, considering the enthalpy of water to be 1,170 BTU, the system operates with an overall thermal efficiency of less than 60 percent. Since, as is noted in the Lockwood patent a pulse jet engine is capable of converting fuel to thermal energy with an efficiency of close to 100 percent, it can be seen that a considerable amount of available energy is not utilized in the prior art system of Lockwood. This inefficiency is especially apparent when it is recognized that the high level, broad band acoustic energy generated by the system also effects moisture removal.
Additionally, when the system disclosed by Lockwood is utilized in processing materials other than fishmeal, it has been found that an excessive amount of material often falls to the floor of the tank. In some situations, this material cannot be removed rapidly enough through openings at the bottom of the tank as Lockwood advocates. In fact, it has been found that such material often builds up to the point where operation of the system must be interrupted and the material manually removed.
Even further, controlling the temperature within the drying tank in the manner taught by the Lockwood patent by varying the rate of fuel flow to the pulse jet engines and varying the pressure within the drying tank is not adequate in all situations. In particular, it can be recognized that the temperature within the drying tank is primarily affected by the amount of moisture being removed. Thus, when the injected material has a high volatile liquid content (e.g., on the order of 75 to 95 percent), the temperature within the tank will rapidly decrease with an increase in material injection rate. This means that, unless substantial temperature control can be effected, the capacity of this system is limited to an injection rate that maintains the temperature throughout various portions of the drying tank within acceptable limits. With the temperature control advanced by Lockwood, it has been found that the system often will not provide a desired operating capability without increasing the size (BTU output) of the pulse jet engines and accepting a further decrease in overall system efficiency. Further, the rather limited temperature control provided by this prior art drying system is not always amenable to the drying of materials having a composition and thermal sensitivty that differs significantly from the fishmeal described in the Lockwood reference. In this respect, although a drying system utilizing pulse jet engines would appear advantageous in processing a wide range of materials, such as animal wastes, municipal and industrial sewage sludge and various waste materials that are the normal by-products of food processing operations, to provide useful products such as fertilizers and animal feed while simultaneously eliminating the discharge of ecologically undesirable effluents, the system disclosed in the Lockwood patent cannot be easily adapted to handle such a wide range of materials on a commercial scale.
Accordingly, it is an object of this invention to provide a drying system that utilizes pulse jet engines wherein the system exhibits relatively high thermal efficiency.
It is another object of this invention to provide a highly efficient, high capacity pulse jet drying system wherein the system is easily adapted to drying a wide range of materials.
It is yet another object of this invention to provide a pulse jet drying system wherein the processed material is continuously removed therefrom without requiring an interruption in system operation.